The present invention relates to an image reading apparatus for acquiring image data used in an image forming apparatus of an electrophotographic type, and also to an image forming apparatus including the image reading apparatus.
An image scanner acquires image data by causing an image reading sensor to execute photoelectric conversion with respect to the reflected light obtained when a read object, such as a sheet-like document, a book, or a three-dimensional object, is irradiated with light.
The image scanner has a transparent document holder (document table) on which an object to be read is held, an illumination device including an illuminating lamp and illuminating the read object placed on the document table, a CCD sensor functioning as an image reading sensor, an optical set (incl. a plurality of mirrors and a reducing glass) located between the document table and the CCD sensor, etc. An analog electrophotographic apparatus, wherein a photosensitive member and a mirror for exposing the image data to the photosensitive member are arranged in place of the CCD sensor and the reflected light from a read object is guided to the photosensitive member, employs a document table (a document holder), an illuminating device and an optical set that are similar in structure to those described above.
With respect to image scanners, it is known that the amount of light emitted therein varies depending upon the deterioration of the illuminating lamp and a change in temperature. In addition, CCD sensors manufactured as products may have different sensitivities from the beginning. Furthermore, pixels also differ in sensitivity. For these reasons, the image data outtput from the CCD sensors is not sufficient for uniform reproduction of image information on a read object. Hence, the image output from the image forming apparatus may be poor in quality.
To provide a solution to this problem, shading data (white reference data and black reference data) is prepared. The data is used as a reference when the CCD sensor converts an image on a read object into image data. The data is based on the optical intensities which the reflected light has when a white reference plate or a black reference plate of a predetermined brightness is irradiated with light. On the basis of the white reference data and black reference data, the image data is subjected to shading correction (i.e., the setting of coefficients for brightness correction). The accuracy of shading correction is of great significance. In many cases, therefore, the data on several lines (the length direction) is averaged in units of one pixel (the width direction) so that dust or foreign matter present (or adhered) around the black reference plate and white reference plate may not be read as an image.
When shading data is obtained, the speed at which the illuminating lamp is moved in the unit time for guiding the reflected light from either the black or white reference plate to the CCD sensor, must be the same as the speed at which the same illuminating lamp is moved in the unit time for actually reading the reflected light (for photoelectric conversion).
In other words, if the amount of reflected light coming from the each of the black reference plate and white reference plate and falling on the pixels varies during the read time (during which the charge produced by the photoelectric conversion of the reflected light falling on the pixels of the CCD sensor is acquired), the reference level of the image data output from the CCD sensor may change.
In many image scanners, the reference plates are arranged in parallel in the direction in which the carriage that holds the illuminating lamp is moved (i.e., the direction in which the document holder extends). This arrangement gives rise to a variety of factors leading to unstable reference data. The factors include the following:
1) In the case where the black reference plate is arranged in front of the white reference plate in a horizontal plane and is therefore irradiated with light before the white reference plate is, the reflected light from the black reference plate is first received, and black reference data is obtained thereby. Immediately after this, the reflected light from the white reference plate is supplied. Hence, the amount of reflected light coming from the white reference plate is inevitably more than the necessary amount at a boundary between the black reference plate and the white reference plate. As a result, an accuracy of the black reference data deteriorates.
2) In the case where the black and white reference plates are arranged in the manner described in 1), the reflected light from the black reference plate is received for a predetermined length of time, and then the reflected light from the white reference plate is supplied. With this in mind, the gain of the CCD sensor is attenuated for reliable detection of the reflected light from the white reference plate. However, if the reflected light from the rear end of the black reference plate falls in the gain-attenuated state of the CCD sensor, the reflected light from the black reference plate is supplied at the boundary though the reflected light from the white reference plate should be supplied there. Accordingly, the accuracy of the white reference data is adversely affected.
3) In the case where the white reference plate is located in front of the black reference plate, the reflected light from the white reference plate is received first, and then the reflected light from the black reference plate is received. In this case, the reflected light from the front end of the black reference plate may fall at the boundary, with the gain attenuated on the condition that the reflected light from the white reference is received. As a result, the accuracy of the white reference data is adversely affected.
4) In the case where the black and white reference plates are arranged in the manner described in 3), the reflected light from the white reference plate is received for a predetermined length of time, and then the reflected light from the black reference plate is supplied. Even if the reception of the reflected light from the black reference plate continues before the sampling reference used by the CCD sensor is changed to a value suited to the black reference plate, the reception takes place, with the gain kept at a low value. As a result, the accuracy of the black reference data is adversely affected.
In a scanner with a high reading rate, the black and white reference plates must be wide (the dimension in the sub scan direction must be increased) as viewed in the direction in which the carriage holding the illuminating lamp moves (i.e., in the direction in which the illuminating lamp moves). This calls for a long distance (a carriage acceleration distance) that enables acceleration of the carriage. The carriage must be accelerated so that it can be moved at the image reading speed before it enters the regions corresponding to the black and white reference plates.
However, if the black and white reference plates that are wide as viewed in the moving direction of the illuminating lamp must be arranged in addition to the document table (which is large enough to hold a maximum-sized read object), then the overall size of the scanner (i.e., the projection area) is inevitably large. This is just the opposite to what is required of recent image scanners, wherein the carriage accelerating distance must be as short as possible to reduce the overall size.
If the carriage accelerating distance is improperly shortened, the carriage must be accelerated rapidly. In this case, it is likely that the carriage will vibrate during acceleration. If the carriage vibrates, the vibration adversely affects the reference data that has effects on the accuracy of the shading correction. If the carriage is accelerated too fast in a scanner wherein the carriage accelerating distance is insufficient, the carriage may reach the leading end of an image before the vibration produced during the acceleration converges (fades away). In this case, the reading of a document image is started when the carriage is still vibrating, and image information at the leading end of the document is hard to read with accuracy.
Another type of scanner is proposed which employ no black reference plate and temporarily stops the light emission from the illuminating lamp. However, temporarily turning off the illuminating lamp has problems in that white reference data cannot be reliably obtained. Specifically, when the white illumination plate is subsequently irradiated with light to obtain white reference data, the white reference data cannot be obtained until the amount of light emitted from the illuminating lamp is saturated.
It follows from this that the time needed before the start of an image on the read object placed on the document table is inevitably increased. Therefore, the total read time needed before the completion of an image increases. In particular, the total read time required when an image of a one-page document is read inevitably increases. This being so, even if an image scanner with a high read rate is used, the number of pages that can be actually read is not large.
A color image scanner is known which decomposes a document image into three primary colors for additive process, namely, R (red), G (green) and B (blue), and outputs three kinds of image data. In this type of scanner, reading is executed by use of a photoelectric conversion element. This element is made up of at least three lines of red (R), green (G) and blue (B); alternatively, it is made up of at least four lines of red (R), green (G), blue (B) and black (K). The memory capacity required may increase, depending upon the size of the reference data generated based on the reflected light from the black and white reference plates.